Issue #3883 - Client: support embedding threadlocal data (e.g. distributed tracing data) into the request.

Added test case that shows how to do it.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

jetty-client/src/test/java/org/eclipse/jetty/client/HttpClientCorrelationDataTest.java

@@ -0,0 +1,70 @@
+//
+//  ========================================================================
+//  Copyright (c) 1995-2019 Mort Bay Consulting Pty. Ltd.
+//  ------------------------------------------------------------------------
+//  All rights reserved. This program and the accompanying materials
+//  are made available under the terms of the Eclipse Public License v1.0
+//  and Apache License v2.0 which accompanies this distribution.
+//
+//      The Eclipse Public License is available at
+//      http://www.eclipse.org/legal/epl-v10.html
+//
+//      The Apache License v2.0 is available at
+//      http://www.opensource.org/licenses/apache2.0.php
+//
+//  You may elect to redistribute this code under either of these licenses.
+//  ========================================================================
+//
+
+package org.eclipse.jetty.client;
+
+import java.io.IOException;
+import java.util.concurrent.TimeUnit;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.eclipse.jetty.client.api.ContentResponse;
+import org.eclipse.jetty.client.api.Request;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ArgumentsSource;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class HttpClientCorrelationDataTest extends AbstractHttpClientServerTest
+{
+    @ParameterizedTest
+    @ArgumentsSource(ScenarioProvider.class)
+    public void testCorrelationData(Scenario scenario) throws Exception
+    {
+        String correlationName = "X-Correlation-Data";
+        String correlationData = "123456";
+        ThreadLocal<String> correlation = new ThreadLocal<>();
+
+        start(scenario, new EmptyServerHandler()
+        {
+            @Override
+            protected void service(String target, org.eclipse.jetty.server.Request jettyRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                assertEquals(correlationData, request.getHeader(correlationName));
+            }
+        });
+        client.getRequestListeners().add(new Request.Listener.Adapter()
+        {
+            @Override
+            public void onQueued(Request request)
+            {
+                request.header(correlationName, correlation.get());
+            }
+        });
+
+        correlation.set(correlationData);
+
+        ContentResponse response = client.newRequest("localhost", connector.getLocalPort())
+            .scheme(scenario.getScheme())
+            .timeout(5, TimeUnit.SECONDS)
+            .send();
+
+        assertEquals(200, response.getStatus());
+    }
+}